Integrated Circuits (ICs) generally comprise many semiconductor features, such as transistors, formed on a semiconductor substrate. The patterns used to form the devices may be defined using a process known as photolithography. Using photolithography, light is shone through a pattern on a mask, transferring the pattern to a layer of photoresist on the semiconductor substrate. The photoresist can then be developed, removing the exposed photoresist and leaving the pattern on the substrate. Various other techniques, such as ion implantation, etching, etc. can then be performed to the exposed portion of the substrate to form the individual devices.
To increase the speed of ICs such as microprocessors, more and more transistors are added to the ICs. Therefore, the size of the individual devices must be reduced. One way to reduce the size of individual features is to use short wavelength light during the photolithography process. According to Raleigh's Law (R=k*λ/NA, where k is a constant, and NA=Numerical Aperture, and R is the resolution of features), a reduction in the wavelength of the light proportionately reduces the size of printed features.
Extreme ultraviolet (EUV) light (e.g., 13.5 nm wavelength light) is now being used to print very small semiconductor features. For example, EUV can be used to print isolated features that are 15–20 nanometers (nm) in length, and nested features and group structures that have 50 nm lines and spaces. EUV lithography is targeted to meet the requirements of a 50 nm half-pitch, where pitch is equal to line plus feature size. Since EUV light has such a short wavelength, it is easily absorbed, even by air. Therefore, EUV photolithography is performed in vacuum using multilayer-coated reflective optics.
EUV photons can be generated by the excited the atoms of a plasma. One way to generate the plasma is to project a laser beam on to a target (droplet, filament jet) creating a highly dense plasma. When the excited atoms of the plasma return to a stable state, photons of a certain energy, and thereby a certain wavelength, are emitted. The target may be, for example, Xenon, Tin, or Lithium. Another way to produce EUV photons is to create a pinch plasma between two electrodes with the target material in a gaseous form between the two electrodes, thereby exciting the atoms.
EUV photons have a very short wavelength. For example, a typical EUV illumination source may generate 13.5 nanometer (nm) photons. The short wavelength of these photons causes the light generated by the illumination source to be easily absorbed, even by air. As a result, mirrors, rather than lenses, are used to focus light generated by EUV illumination sources.
A dense plasma focus (DPF) electrode may be used to generate EUV photons. The DPF electrode includes an anode, a cathode, and a plasma disposed between the anode and the cathode. When an arc is generated between the anode and the cathode, the individual atoms of the plasma are excited and generate EUV photons.
A DPF electrode may generate physical debris because of Brownian movement. To protect the focusing mirrors, a disc-shaped foil obscuration is placed in front of the electrode. However, the obscuration absorbs a large portion of the relatively little light energy generated by the DPF electrode.